Protective and monitoring systems of the above type are often used in places where a moving range of a constructional element of a machine or device must be monitored to avoid collisions. Typical applications are found in passageways or driveways provided with automatically closing and/or motor-driven industrial doors.
A large variety of such monitoring systems are described in the prior art. DE 20 2008 009 320 U1, for example, describes a monitoring device comprising an optical protective device having a monitoring sensor for avoiding, for example, danger caused by moving machine parts in the door area. DE 37 28 354 describes protective systems for door arrangements, the systems comprising a transmission unit, a reception unit, a control unit, and light sources.
Due to their robust nature and their easy operability, such monitoring devices are often equipped with optical detection devices for detecting possible obstacles. These detection devices usually comprise a transmission unit and a reception unit, wherein an optical signal, for example, a light beam, is transmitted therebetween. A danger zone where a machine movement takes place can thereby be continuously monitored. When such a signal is interrupted by an obstacle in the danger zone, the control device will output a triggering pulse for stopping the machine movement.
A known problem in such detection devices resides in the occurrence of extraneous light or other situations causing a change of the light path, with a resultant risk of faulty detection or non-detection of a light signal. Such situations result, for example, from the fact that, due to their constructional design, the transmission unit and the reception unit will involve a certain lateral radiation of the transmitted and/or received optical signal, comparable to a headlight generating a light cone with a radiation angle. Prior art devices are therefore normally provided with certain constructional measures on the transmission and reception units, such as, for example, arrangement of a plurality of separate transmission/reception units, a relatively small radiation angle, and/or a relatively exact mutual alignment.
This has the disadvantage, however, that such devices are restricted in their availability and in their functionality, and are expensive to produce and install.